The present invention relates to a method and apparatus for the control of microbial contaminants in liquids and, more particularly, to the destruction of such contaminants in shipboard fuels and ballast waters by the use of ultrasonic vibration to cause cavitation within these liquids, that apparatus being designed to prevent cavitation from damaging parts of the apparatus that create the ultrasonic vibrations.
Microbiological contamination of hydrocarbon fuels presents a variety of problems to the operators of naval vessels. Some of the organisms responsible for such contamination are fungi, yeast and bacteria.
In naval vessels, it is common for water to be found in on board fuel tanks. This water originates from various sources such as condensation from the fuel, water leakage into the fuel or from water taken on as ballast in the tanks. The presence of water in the fuel tank results in the proliferation of yeasts and fungi at the fuel/water interface where the microbial contaminants extract oxygen from the water and nutrients from the fuel layer. Some forms of these microorganisms produce water as a byproduct, thereby altering the environment of the fuel/water interface and allowing other microbial forms to flourish.
Various problems arise from the microbiological contamination of fuel including:
(a) Mat-like or slimy deposits at the fuel/water interface;
(b) Blockages of valves, pumps, filters and coalescers;
(c) Reduction in interfacial tension resulting in the malfunction of water separating devices;
(d) Accelerated corrosion of steel and aluminum;
(e) Black stains on copper alloys or silver plated components;
(f) Injector fouling; and
(g) Probe fouling and incorrect volume measurement.
Some of these problems have previously been documented (R. D. Haggett and R. M. Morchat, Intl. Biodeterioration and Biodegradation 29 (1992) 87-99).
These consequences can be tolerated at minor levels of infection. However, as the microbial population flourishes, serious and costly failures are inevitable. Generally, contamination problems are only investigated when the failure or malfunction of equipment occurs. Fuel tanks, and associated systems, found to contain such contaminants must be drained, cleaned, dried and inspected prior to being reused.
Completely sterile natural environments are rare and without strong chemical additives toxic to microbes, some level of contamination can always be expected. However, if the levels of this contamination can be kept below critical levels, their proliferation can be prevented and the damaging consequences avoided.
The only means of controlling microbiological contamination in ship board fuel systems at present is to prevent water from accumulating in fuel tanks, which is extremely difficult and impractical, or to treat the contaminated fuel with biocidal agents. However, the use of such biocides presents environmental and health and safety concerns. Questions have arisen concerning the effect of biocide containing fuel on personnel working daily with fuel system components as well as personnel working in confined spaces where they may be exposed to vapours containing the biocide. The environmental concern relates to the effect that such biocidal agents may have if introduced into already sensitive marine ecosystems. The selective nature of biocides presents a further problem in their usage. For example, while some biocides are effective against fungi they have little or no effect on bacteria. Further, while some biocides inhibit growth of pure microbial cultures, their effectiveness is drastically reduced when applied to mixtures of fungi, yeasts and bacteria.
The use of ultrasound as a germicidal agent has been investigated previously by G. Scherba et al (Applied and Environmental Microbiology 1991, 2079-2084) and H. Kinsloe et al (J. Bacteriology 68 (1954) 373-380). The literature on the treatment of microorganisms using ultrasonics is sparse, but all studies that have been carried out agree that it is an effective means of destroying microorganisms. A shipboard application of this technology is waste water treatment. This possibility was studied by the U.S. Navy Coastal Systems Station in 1976 (A. J. Ciesluk, xe2x80x9cAcoustic Sterilization For Shipboard Waste Managementxe2x80x9d, U.S. Navy Coastal Systems Station Technical Report, NCSC-TR-329-78). In this study, two commercial ultrasonic cleaners were used at two different power levels. However, it was concluded that the basin volumes of these cleaners were too large to lead to effective cell disruption. That literature does not describe the use of ultrasound to control microbial populations in fuel systems although the possibility has been proposed (E. C. Hill (1986), xe2x80x9cMicrobial Problems In Offshore Oil Industryxe2x80x9d Proceedings of the International Conference, Inst. Petroleum Microbiology Committee, Aberdeen, U.K.).
Because of its inherent safety and relatively low power requirements compared to other physical control measures, ultrasound may represent the ideal solution to microbiological contamination of fuel systems. If the fuel and/or the water in the vicinity of the fuel/water interface is treated on an ongoing basis, the microbial populations can likely be kept below critical levels. This would represent a more environmentally friendly and more effective control measure than the biocides currently in use.
Accordingly, it is an object of the present invention to overcome the limitations of known fuel decontamination methods and provide a safe and effective process for the control of microbial populations in fuel systems. It is also an object of the present invention to provide a system and a process for the effective treatment of microbiologically contaminated ballast waters prior to disposal thereof.
Specifically, the present invention is directed to an apparatus for neutralizing microbiological contamination of a liquid fuel comprising subjecting the fuel to ultrasonic vibrations in order to cause cavitation within the liquid and, thereby, to destroy the microbial contaminants and which is designed to avoid any cavitation from damaging the parts of the apparatus that create the ultrasonic vibrations.
An apparatus for the ultrasonic treatment of a microbiologically contaminated liquid, according to one embodiment of the invention, comprises:
a module having a treatment container and an ultrasonic generating means for subjecting ultrasonic vibrations on the liquid in the container where contaminated liquid in the container subjected to the ultrasonic vibrations result in cavitation in the liquid and the destruction of microorganisms contained therein, the ultrasonic vibration generating means being located outside of the container and submerged in a transmission fluid pressurized to prevent cavitation occurring at areas surrounding the ultrasonic generating means.